The Lassen site is considered to be a clean air site in California where anthropogenic influence is at a minimum. The sampling was conducted from 18:10 April 15th to 21:10 May 24th in 2002.The DRUM sampler had eight size cuts: Inlet-5 um, 5-2.5 um, 2.5-1.15 um, 1.15-0.75 um, 0.75-0.56 um, 0.56-0.34 um, 0.34-0.26 um, 0.26-0.09 um. Half of the collection strip width was covered by aluminum on top of Mylar providing two different substrates for aerosol collection.
The aluminum strips were analyzed in the TOF MS system and Mylar strips were used in S-XRF analyses. The S-XRF was performed on Beamline 10.3.1 at the Advanced Light Source (ALS), Lawrence Berkeley National Lab (LBNL). S-XRF quantifies elements from sodium through uranium. A custom built time-of-flight mass spectrometer was used to obtain laser desorption ionization mass spectra to provide relative quantification of molecular species as a function of position on the sample strip. Peak areas of each species detected are normalized to the total sulfur concentration from S-XRF data. The aluminum sample strip was attached to a translation stage in the mass spectrometer source region, and pulsed 266nm radiation was used to desorb and ionize the collected aerosol. The negative ion spectra contain ion fragments due to the use of higher laser power required for negative ion formation. Intensity as a function of position on the strip is converted to intensity versus time using the known rotation rate of the drum collection media.
A diurnal cycle of daytime up slope wind and nocturnal down slope/stable air is observed during April 18-26 on Lassen Peak. The methyl sulfonate (MSA)/bisulfate ratio follows the diurnal cycle due to condensation of sulfate species with increasing nocturnal humidity and the presence of the CCN sulfates in the upper airmass. The MSA is a marker for oxidation of DMS from the ocean, the correlation with stage 8 soil indicates both are from long range transport. The general decrease in MSA/sulfate ratio maybe from either catalysis of the DMS oxidation process by the metal oxides in soil favoring sulfate formation, and/or an increase of (anthropogenic) sulfate associated with the ultra fine soil long range transport.Ultrafine soil correlates with sulfur during the April event, entirely counter to coarse soil which peaks in association with the day time upslope winds. Coarse soil originates from local sources. The temporal behavior and correlation with MSA indicate that ultrafine soil is due to long range transport. Asian dust was observed at Trinidad Head on April 22,24 and 26 [Perry, 2004]. The observation of Asian ultrafine soil aerosol in in accordance with previous work at Lassen Peak [Van Curren, 2003]
The spectra of the sub-micron size fractions were dominated by carbon cluster, sulfate, and methyl sulfonate (MSA) signals. There were additional signals from iodine, and signals from organic acids, notably bioxalate. The appearance of MSA, iodine, and oxalate suggest a marine or oceanic component of the aerosol. The spectra are very similar in the sub-micron fraction for both sample sets. It is hypothesized that the iodine is present as iodate rather than HI since HI would be removed from an acidic aerosol while iodate has significantly lower vapor pressure.
The 2.5 1.1 micron size fraction differs chemically from the smaller size fractions. The larger aerosols are dominated by nitrate. The fractionation of the volatile nitric acid component from the sub-micron fraction to the less acidic micron size fraction is similar to the chemistry of the coastal Antarctic aerosol. The elemental carbon signal remains ubiquitous in the aerosol spectra.
Organic acids and dicarboxylic acids show strong correlation with MSA indicating that the organic compounds and MSA appear due to long range transport, and probably have a common source. MSA and the organic acids are typically found from oceanic sources. The principle natural source of organic acids is oxidation of fatty acids. Degradation of combustion exhaust is another possible source of these acids associated with long range transport, however the correlation with MSA strongly suggests an oceanic source.
Conclusion The total aerosol concentration from April 19th to April 23rd exhibits a clear daily cycle. The ultrafine soil fraction correlates with sulfur during the April event, entirely counter to the coarse soil fraction which peaks in association with the daytime upslope wind. The coarse soil fraction is local. Both MSA and sulfate peak at night due to lower temperatures and high relative humidity associated with the nocturnal down slope/stable wind. The general decrease in MSA/sulfate ratio maybe from either catalysis of the DMS oxidation process by the metal oxides in soil favoring sulfate formation, and/or an increase of (anthropogenic) sulfate associated with long range transport of ultra fine soil. Chemical analysis shows oceanic influence through strong correlations for methyl sulfonic acid (MSA), iodine, and oxalate. The appearance of the oceanic biogenic tracers in the sub-micron fraction is most likely a result of vertical mixing over the Pacific Ocean.
Acknowledgements: This study is supported by NOAA, grant GC02-148.
VanCuren, Richard A.. Asian aerosols in North America: extracting the chemical composition and mass concentration of the Asian continental aerosol plume from long-term aerosol records in the western United States. Journal of Geophysical Research, [Atmospheres] (2003), 108(D20).
Perry, Kevin D.; Cliff, Steven S.; Jimenez-Cruz, Michael P. Evidence for hygroscopic mineral dust particles from the Intercontinental Transport and Chemical Transformation ExperimentJ. Geophys. Res., Vol. 109, No. D23, D23S28, 10.1029/2004JD004979.
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